In-mold expanding molded products, which are produced by (i) introducing, into a mold for in-mold expanding molding, polyolefin resin expanded particles that have been produced by expanding polyolefin resin particles and (ii) fusing the polyolefin resin expanded particles by heating, are used in various ways such as packaging materials, buffer materials, heat insulators, building materials, and automobile materials.
In some fields such as building materials and automobile materials among those uses, in-mold expanding molded products are required to have flame retardancy. Therefore, there are proposed polypropylene resin expanded particles made from resin containing a flame retardant and a flame retardant aid as necessary and in-mold expanding molded products made from the polypropylene resin expanded particles (see, for example, Patent Literatures 1 and 2).
Colored polyolefin resin expanded particles or in-mold expanding molded products made from the colored polyolefin resin expanded particles are demanded in some cases in view of appearance. Particularly in the field of automobile parts etc., black-colored polyolefin resin expanded particles or black-colored in-mold expanding molded products are demanded, and carbon black is generally used as a black pigment for in-mold expanding molded products because of its tinting strength, heat resistance, etc.
Meanwhile, weight reduction of those materials have been carried out in recent years, and requests for expanding molded products having a high expanding ratio have been increased accordingly.
Automobile interior part materials etc. usually must satisfy flammability standards, and there are therefore demands for flame-retardant materials or materials having a slow flame velocity. However, in-mold expanding molded products in which carbon black is used and in-mold expanding molded products having a high expanding ratio tend to be flammable. Therefore, in many cases, black in-mold expanding molded products having a high expanding ratio cannot satisfy the flammability standards.
Various methods of preparing nonflammable polyolefin resin which is originally flammable are examined, and addition of a flame retardant is a general method. As the flame retardant added to polyolefin resin, the following various flame retardants are used: halogen-containing compounds, hydrated metal oxides, phosphate esters, and nitrogen-containing compounds, and such examples of flame retardants used for polyolefin resin expanding bodies are disclosed in Patent Literatures 1 to 3 typically. In recent years, non-halogen flame retardants are preferably used because of, for example, an environmental problem that halogen-containing compounds may generate a harmful gas at the time of combustion. In addition, those non-halogen flame retardants are used in large amounts for resin, which may cause reduction in mechanical properties and deterioration of moldability.
Further, in recent years, sterically hindered amine ether flame retardants are proposed to be used as non-halogen flame retardants for polypropylene resin in-mold expanding molded products (see Patent Literatures 4 and 5). However, in a case where sterically hindered amine ether flame retardants are used in polypropylene resin in-mold expanding molded products containing carbon black, the sterically hindered amine ether flame retardants have insufficient flame retardancy in some cases. Those organic compound molecule type flame retardants are extremely expensive, but may have a low heat resistance, and may limit process conditions.
The step of molding polyolefin resin expanded particles into a polyolefin resin in-mold expanding molded product is also carried out as follows in many cases: air is pressurized in the polyolefin resin expanded particles in advance in a pressure-resistant vessel to impregnate air to the polyolefin resin expanded particles, thereby imparting an expandability; an in-mold expanding molding apparatus is filled with the polyolefin resin expanded particles; and the polyolefin resin expanded particles are heated with use of heated steam.
In actual production, however, change in internal pressure of polyolefin resin expanded particles because of air impregnation, reduction of the internal pressure after removal of the polyolefin resin expanded particles from the pressure-resistant vessel but before introduction of the polyolefin resin expanded particles into the in-mold molding apparatus, and variation of the internal pressure of the polyolefin resin expanded particles occur in some cases. It was found that the internal pressure of the polyolefin resin expanded particles is extremely decreased depending on which kind of flame retardant is used with the polyolefin resin expanded particles, which easily results in generation of wrinkle on a surface of a molded product thus produced and in occurrence of gaps between the polyolefin resin expanded particles. Accordingly, even if the internal pressure of the expanded particles varies, a favorable molded product is desirably produced.